1. Field of the Invention
The present invention relates to a control scheme for a power supply, and more specifically to a time delay synchronous control scheme for a power supply, which has multiple outputs and tight output regulations.
2. Description of the Related Art
In the design of switching mode power supplies (SMPS) with multiple outputs, it is important to meet the requirements for the cross regulation of the multiple outputs. Additionally, with the development of the SMPS, it is imperative to consider high efficiency and low cost.
FIG. 1 illustrates a block diagram of the conventional implementation for SMPS with multi-outputs. This implementation includes a front-end DC/DC converter 1 and is cascaded with two post buck converters 2 and 3. One of the outputs is achieved as Vo3 directly from the front-end DC/DC converter 1. Both of the post buck converters 2 and 3 have an input filter with a LC structure, cascading the output capacitor Cf of the front-end DC/DC converter 1.
The conventional SMPS as shown in FIG. 1 can achieve tight multiple output regulations because the front-end DC/DC converter 1 and the post buck converters 2 and 3 operate respectively and independently. However, the additional cost of the input filters of the post buck converters 2 and 3 are not what was anticipated. Furthermore, although the input filter is added for each of the post buck converters 2 and 3, a ripple current with a considerable rms (root mean square) value is still obtained on the output capacitor Cf of the front-end DC/DC converter 1 because of the inherent pulsating input traits of the post buck converters 2 and 3. This will significantly increase the current stress and the loss dissipated on the output capacitor. To alleviate the pressure due to the loss rise, the use of a higher number of capacitors or alternatively, high quality capacitors are inevitable. This will lead to a higher cost for this architecture, especially in the case when the outputs have large output currents.
It is therefore attempted by the applicant to provide a new architecture with a new control scheme, which can overcome the defects of the prior art for SMPS with multiple output application.